Direct conversion of electricity into light using semiconductor-based light-emitting diodes (LEDs) is widely accepted one of the most promising approaches to more efficient lighting. LEDs demonstrate high brightness, long operational lifetime, and low energy consumption performance that far surpass that of conventional lighting systems such as incandescent and fluorescent light sources. The LED field is currently dominated by semiconductor quantum-well emitters (based, e.g., on InGaN/GaN) fabricated by epitaxial methods on crystalline substrates (e.g., sapphire). These structures are highly efficient, reliable, mature and bright, but structural defects at the substrate and semiconductor interface caused by lattice mismatch and heating during operation generally limits such devices to point light source with limited flexible compatibility.
OLEDs are easily amendable to low-temperature, large-area processing, including fabrication on flexible substrates. Synthetic organic chemistry provides essentially an unlimited number of degrees of freedom for tailoring molecular properties to achieve specific functionality, from selective charge transport to color-tunable light emission. The prospect of high-quality lighting sources based on inexpensive “plastic” materials has driven a tremendous amount of research in the area of OLEDs, which in turn has led to the realization of several OLED-based high-tech products such as flat screen televisions and mobile communication devices. Several industrial giants such as Samsung, LG, Sony, and Panasonic are working to develop large-area white-emitting OLEDs both for lighting and display. Despite advances in the OLED field, there are a few drawbacks of this technology that might prevent its widespread use in commercial products. One problem is poor cost-efficiency caused at least in part by the complexity of the necessary device architecture, which requires multiple thermal deposition steps during manufacture. Another problem is their limited stability, particularly for deep-red and blue phosphorescent OLEDs. While improving greatly in recent years, they still do not meet the standards employed in high-end devices.
Chemically synthesized nanocrystal quantum dots (QDs) have emerged as a promising class of emissive materials for low-cost yet efficient LEDs. These luminescent nanomaterials feature size-controlled tunable emission wavelengths and provide improvements in color purity, stability and durability over organic molecules. In addition, as with organic materials, colloidal QDs can be fabricated and processed via inexpensive solution-based techniques compatible with lightweight, flexible substrates. Moreover, similar to other semiconductor materials, colloidal QDs feature almost continuous above-band-edge absorption and a narrow emission spectrum at near-band-edge energies. Distinct from bulk semiconductors, however, the optical spectra of QDs depend directly on their size. Specifically, their emission color can be continuously tuned from the infrared (IR) to ultraviolet (UV) by varying QD size and/or composition. The wide range spectral tunability is combined with high photoluminescence quantum yields that approach unity in well-passivated structures. These unique properties of QDs have been explored for use in various devices such as LEDs, lasers, solar cells, and photo detectors.
Nonetheless, semiconductor-based LEDs remain the most widely used solid state replacement light source. Retrofit of traditional lighting devices such as indoor lamps (bulb, tube, spot light, down light, etc.), professional luminaires (torches, table lamps, track light, etc.) and outdoor road lights with these LED-based solutions continues due to the energy savings, long lifetime, digitalization and cost reduction provided by LED as compared to that of traditional incandescent bulb, fluorescent and high-intensity discharge light sources.